If you've taken a high-school biology class or have friends who are bodybuilders, you've probably heard this fact: Amino acids are the building blocks of protein.

It's true that amino acids are the foundation of protein—one of the three macronutrients, along with fats and carbohydrates, that make up the bulk of the human diet. And yes, bodybuilders love aminos, since protein is critical for building muscle mass.

Even those among us who don't aspire to get ripped can benefit from amino acids. And there's a reason nine of them are called "essential" amino acids: These organic molecules obtained from protein-containing foods are crucial to countless biological processes in our bodies, including giving cells their structure, forming organs and muscles, breaking down food, repairing tissue, producing energy, and more.

There are a total of 20 amino acids that human bodies require to produce all the proteins needed to function and grow. Here's what you need to know about them—including the benefits of the nine amino acids that are considered "essential."

Essential versus nonessential amino acids.

"When we consume proteins through food, our body breaks them back down into amino acids, which can be reused to make the proteins the body needs," Lisa Hayim, M.S., R.D., NYC-based registered dietitian and founder of The Well Necessities, told mbg. "It may be helpful to think of amino acids as the cars on the train. Each car is an amino acid, yet the whole train is the protein."

While you need all 20 amino acids to function optimally, some of these are produced naturally by your body, making them nonessential amino acids. This means you don't need to obtain them from the foods in your diet.

The nine essential amino acids, on the other hand, must always be obtained from food. Your body uses essential amino acids to produce nonessential and conditionally essential amino acids, which makes them of vital importance.

The 9 essential amino acids: what they do & what foods to eat.

Each of the nine essential amino acids—histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine—has unique properties. For example, while some essential amino acids are extra important for muscle development, others play a greater role in collagen production or regulating mood. This means it may be particularly beneficial for you to seek certain ones depending on your individual needs. For health conditions including diabetes and anxiety, doctors have had success treating patients with therapeutic doses of specific amino acids.

Although animal proteins such as beef, eggs, fish, dairy, and poultry contain good amounts of all nine essential amino acids (making them "complete" proteins), you can also obtain essential amino acids from plant-based foods. While it's true that most plant foods don't contain all nine essential amino acids (or at least not in adequate proportions), vegetarians and vegans can ensure healthy intake by consuming a variety of plant protein sources over the course of a day. You can search for foods that contain specific amino acids by using the USDA Food Composition Database.

Here is more on each of the nine essential amino acids, including the main roles each one plays in the body and where to find them in your favorite foods:

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1.
Histidine

The essential amino acid histidine is needed for the growth and repair of tissue, particularly for the maintenance of myelin sheaths—sleeves of fatty tissue that protect nerve cells, ensuring that they're able to send and receive messages. It even helps protect tissues against damage caused by radiation, and functions as a chelating agent to remove heavy metals from the body.

Histidine has antioxidant and anti-inflammatory properties and plays a role in reducing rheumatoid arthritis symptoms. It's a precursor to the neurotransmitter histamine, which plays a vital role in immune functioning and helps produce red and white blood cells.

One important note: Adults can typically produce enough histidine from other amino acids, but children need to obtain histidine from food. This makes it sort of unique in that it's both essential and nonessential, depending on your age.

2.
Isoleucine

The essential amino acid isoleucine is one of three branched-chain amino acids (BCAAs), along with leucine and valine, that the body uses for muscle repair and growth. It's heavily concentrated in the muscle tissue and plays an important role in muscle metabolism, providing your muscles with the appropriate fuel to do work. Isoleucine deficiency can result in muscle spasms.

3.
Leucine

The essential amino acid leucine is one of three branched-chain amino acids (BCAAs) that the body uses for muscle repair and growth. In fact, it's often considered the most important amino acid for building muscle mass. That's partly because leucine appears to be the main amino acid responsible for activating mTOR (mammalian target of rapamycin), a signaling pathway that's responsible for stimulating protein synthesis.

Lysine is also crucial for the production of collagen—the most abundant protein in the body that gives structure to ligaments, tendons, skin, hair, nails, cartilage, organs, bones, and more. Experts suggest that lysine, along with vitamin C and the amino acid proline, are essential for the formation of healthy collagen. Together, they form procollagen, which is then converted into several different types of collagen found in various tissues throughout the body.

5.
Methionine

The essential amino acid methionine is a sulfur-containing compound. The sulfur provided by methionine aids in several detoxifying processes such as protecting cells from pollutants, slowing cell aging, and absorbing selenium and zinc. As a sulfur-containing amino acid, methionine also improves the tone and elasticity of skin and strengthens hair and nails.

6.
Phenylalanine

The essential amino acid phenylalanine plays a key role in the creation of other amino acids, including tyrosine. Tyrosine, in turn, has a number of uses in the body, including the production of the neurotransmitters dopamine, norepinephrine (noradrenaline), and epinephrine (adrenaline)—and thus plays a role in regulating mood and emotional response, as well the body's fight-or-flight response.

Tyrosine also helps produce melanin, which is what gives our skin, hair, and eyes their color, and it contributes to thyroid health—the thyroid must combine tyrosine and iodine to make thyroid hormone, which helps regulate metabolism. (Think you might have a thyroid imbalance? Read this.)

7.
Threonine

The essential amino acid threonine plays a central role in the production of collagen and elastin, which help provide structure and stretchiness to skin and connective tissues. It's also found in high concentrations in the central nervous system, and some research suggests it may help reduce symptoms of spasticity (when certain muscles are continuously contracted) in multiple sclerosis patients, as well as alleviate anxiety and mild depression.

Threonine is important for maintaining a healthy gut and digestive tract as well. It's needed to produce the mucus layer that covers and protects the digestive tract, and without it, animal research reveals that the gut lining could become compromised. This, in turn, could put you at risk for a variety of autoimmune conditions. Additionally, threonine is important for fat metabolism and helps prevent fat buildup in the liver.

8.
Tryptophan

The essential amino acid tryptophan is a precursor to serotonin, a feel-good neurotransmitter essential in regulating appetite, sleep, mood, and pain, and which has natural sedative effects. Serotonin has actually been shown to reduce appetite, meaning that consuming enough tryptophan-containing foods could potentially aid in weight loss.

Research has found tryptophan to be effective at relieving symptoms of premenstrual syndrome, while low levels have been associated with mood swings, anxiety, and depression. This essential amino acid also supports the production of niacin (vitamin B3), which is involved in metabolism and is a precursor to melatonin, a hormone that regulates sleep and wake cycles—which is exactly why everyone says you feel sleepy after a big Thanksgiving dinner with lots of tryptophan-rich turkey.

9.
Valine

The essential amino acid valine is one of three branched-chain amino acids (BCAAs), along with isoleucine and leucine, that the body uses for muscle repair and growth. Like the other two BCAAs, valine also helps regulate blood sugar and maintain energy levels by supplying glucose to muscles during workouts.

Valine also has stimulant activity and has been said to help maintain mental and physical stamina, while its role in the central nervous system supports emotional calm. Valine has also been shown to be a useful supplemental therapy in treating liver disease.

Do you need to supplement with essential amino acids?

If not, then you may not be getting adequate levels of the nine essential amino acids. In that case, start eating more protein from a variety of plant and animal sources, like the ones mentioned above. If you're a vegetarian or vegan, or anyone who's not confident that your diet is quite cutting it in the protein department, consider supplementing with a quality protein powder, which can easily be added to smoothies, oatmeal, and baked goods.

Whey protein is a naturally complete protein and contains an adequate proportion of each of the nine essential amino acids. Vegan protein powders are typically always complete, too, since they're often made with a variety of different plant proteins (such as a combo of pea, hemp, and brown rice proteins) to cover all your bases, or they're made from soy, one of the few complete plant-based proteins. If you choose a soy protein, just make sure it's organic.

Bottom line, though, is that you can definitely obtain healthy levels of all nine essential amino acids in a healthy, varied diet—regardless of whether that diet contains animal products.

What about branched-chain amino acids (BCAAs)?

If you're at all into fitness, you've probably heard of branched-chain amino acids, or BCAAs. These are the three essential amino acids—leucine, isoleucine, and valine—that seem to be extra important for maintaining muscle. Research shows that they activate key enzymes that promote muscle growth. The "branched chain" refers to their chemical structure.

Yet, BCAAs may serve other purposes, too. Some experts tout their fatigue-fighting benefits, as research shows they can interfere with the transport of the relaxation-inducing amino acid tryptophan, thus preventing you from getting too sleepy.

The good news: Unless your health care provider suggests otherwise, there's no need to take a BCAA supplement if you're eating well. After all, BCAAs are already found in the animal and plant foods listed in this article for leucine, isoleucine, and valine. Additionally, Hayim says that whey protein is one of the best sources of all three of these BCAAs.